Gas separation processes using membranes have undergone a major evolution since the introduction of the first membrane-based industrial hydrogen separation process about two decades ago. The disclosure of new materials and efficient methods for making membranes will further advance the membrane gas separation processes within the next decade.
The gas transport properties of many glassy and rubbery polymers have been measured as part of the search for materials with high permeability and high selectivity for potential use as gas separation membranes. Unfortunately, an important limitation in the development of new membranes for gas separation applications is a well-known trade-off between permeability and selectivity of polymers. By comparing the data of hundreds of different polymers, Robeson demonstrated that selectivity and permeability seem to be inseparably linked to one another, in a relation where selectivity increases as permeability decreases and vice versa.
Despite concentrated efforts to tailor polymer structure to improve separation properties; current polymeric membrane materials have seemingly reached a limit in the trade-off between productivity and selectivity. For example, many polyimide and polyetherimide glassy polymers such as Ultem® 1000 have much higher intrinsic CO2/CH4 selectivities (αCO2/CH4)(˜30 at 50° C. and 690 kPa (100 psig) pure gas tests) than those of polymers such as cellulose acetate (˜22), which are more attractive for practical gas separation applications. These polyimide and polyetherimide glassy polymers, however, do not have permeabilities attractive for commercialization compared to current commercial cellulose acetate membrane products. On the other hand, some inorganic membranes, such as SAPO-34 and DDR zeolite membranes and carbon molecular sieve membranes, offer much higher permeability and selectivity than polymeric membranes for separations, but are too brittle, expensive, and difficult for large-scale manufacture. Therefore, it remains highly desirable to provide an alternate cost-effective membrane with improved separation properties compared to the polymer membranes.
Based on the need for a more efficient membrane, a new type of membrane, mixed matrix membrane (MMM), has been developed. MMMs are hybrid membranes containing inorganic particles such as molecular sieves dispersed in a continuous polymer matrix.
MMMs have the potential to achieve higher selectivity and/or greater permeability compared to the existing polymer membranes, while maintaining their advantages such as low cost and easy processability. Much of the research conducted to date on MMMs has focused on the combination of a dispersed solid molecular sieving phase, such as molecular sieves or carbon molecular sieves, with an easily processed continuous polymer matrix. For example, see U.S. Pat. No. 6,626,980; US 2005/0268782; US 2007/0022877; and U.S. Pat. No. 7,166,146. The sieving phase in a solid/polymer mixed matrix scenario can have a selectivity that is significantly larger than the pure polymer. Therefore, in theory the addition of a small volume fraction of molecular sieves to the polymer matrix will significantly increase the overall separation efficiency. Typical inorganic sieving phases in MMMs include various molecular sieves, carbon molecular sieves, and traditional silica. Many organic polymers, including cellulose acetate, polyvinyl acetate, polyetherimide (commercially Ultem®), polysulfone (commercial Udel®), polydimethylsiloxane, polyethersulfone, and several polyimides (including commercial Matrimid®), have been used as the continuous phase in MMMs.
Most recently, significant research efforts have been focused on materials compatibility and adhesion at the inorganic molecular sieve/polymer interface of the MMMs in order to achieve separation property enhancements over traditional polymers. For example, Kulkami et al. and Marand et al. reported the use of organosilicon coupling agent functionalized molecular sieves to improve the adhesion at the sieve particle/polymer interface of the MMMs. See U.S. Pat. No. 6,508,860 and U.S. Pat. No. 7,109,140. This method, however, has a number of drawbacks including: 1) prohibitively expensive organosilicon coupling agents; 2) very complicated time consuming molecular sieve purification and organosilicon coupling agent recovery procedures after functionalization. Therefore, the cost of making such MMMs having organosilicon coupling agent functionalized molecular sieves in a commercially viable scale can be very expensive. Most recently, Kulkami et al. also reported the formation of MMMs with minimal macrovoids and defects by using electrostatically stabilized suspensions. See US 2006/0117949. US 2005/0139065 A1 to Miller et al., entitled “Mixed matrix membranes with low silica-to-alumina ratio molecular sieves and methods for making and using the membranes”, reports the incorporation of low silica-to-alumina (Si/Al) ratio molecular sieves into a polymer membrane with a Si/Al molar ratio of the molecular sieves preferably less than 1.0. Miller et al. claim that when the low Si/Al ratio molecular sieves are properly interspersed with a continuous polymer matrix, the MMM ideally will exhibit improved gas separation performance.
While the polymer “upper-bound” curve has been surpassed using solid/polymer MMMs, there are still many issues that need to be addressed for large-scale industrial production of these new types of MMMs. One feature that needs improvement is the excessive thickness of the MMMs. Most of the molecular sieve/polymer MMMs reported in the literature are in the form of thick symmetric mixed matrix dense films with a thickness of about 50 μm and molecular sieve particles with relatively large particle sizes in the micron range have been used. Commercially available polymer membranes, such as cellulose acetate and polysulfone membranes, however, have an asymmetric membrane structure with a less than 500 nm thin dense selective layer supported on a porous non-selective layer. As a consequence, the dense selective layer thickness of the mixed matrix membranes is much thinner than the particle size of the molecular sieve particles. Voids and defects, which result in reduced overall selectivity, are easily formed at the interface of the large molecular sieve particles and the thin polymer matrix of the asymmetric MMMs. Therefore, controlling the thickness of the thin dense selective mixed matrix membrane layer and the particle size of the molecular sieve particles is critical for making large scale asymmetric MMMs with at least 20% increase in selectivity compared to the corresponding asymmetric polymer membranes containing no molecular sieves.